1. Field of the Invention
The present invention relates to a spontaneous light emitting device, in particular, an active matrix type spontaneous light emitting device. Further, in particular, the present invention relates to an active matrix type spontaneous light emitting device using a spontaneous light emitting element including an organic electroluminescence (EL) element for a pixel portion. The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.
2. Description of the Related Art
In recent years, an active matrix type spontaneous light emitting device using a spontaneous light emitting device in which a semiconductor thin film is formed on an insulating body such as a glass substrate or the like, in particular, TFT has remarkably come into wide use. The active matrix type spontaneous light emitting device using the TFTs has hundreds of thousands to millions of TFTs in the pixel portion arranged in a matrix and displays an image by controlling the charges of the respective pixels.
A technology relating to a polysilicon TFT for forming a driving circuit at the same time by using a TFT around a pixel portion in addition to a pixel TFT constituting an pixel has been developed as a recent technology and contributes to the miniaturization and low power consumption of the device and hence the spontaneous light emitting device becomes an indispensable device for the display unit of a mobile gear which has been remarkably expanded in the application in recent years.
The spontaneous light emitting device utilizing a spontaneous light emitting material such as an organic EL and the like has received widespread attention as a flat display substituting for a LCD (liquid crystal display) and has been actively researched.
In FIG. 15A is schematically shown a conventional spontaneous light emitting device. In the present specification, an organic EL (hereinafter simply referred to as “EL”) will be described as an example of a spontaneous light emitting device. A pixel portion 1504 is arranged in the center of a substrate 1501 made of an insulating material (for example, glass). In the pixel portion 1504 are arranged electric current supply lines 1505 for supplying an electric current to EL elements in addition to source signal lines and gate signal lines. On the upper side of the pixel portion 1504 is arranged a source signal line driving circuit 1502 for controlling the source signal lines, and on the right and left sides are arranged gate signal driving circuits 1503 for controlling the gate signal lines. In this connection, in FIG. 15A, the gate signal line driving circuits 1503 are arranged on both the right and left sides of the pixel portion but the gate signal line driving circuit 1503 may be arranged only on one side. However, it is desirable from the viewpoint of driving efficiency and reliability that the gate signal line driving circuits 1503 are arranged on both sides. Signals are applied to the source signal line driving circuit 1502 and the gate signal driving circuits 1503 from the outside via a flexible printed circuit board (FCP) 1506.
An enlarged view of a portion surrounded by a dotted line 1500 in FIG. 15A is shown in FIG. 15B. In the pixel portion, as shown in this figure, respective pixels are arranged in a matrix. Further, in FIG. 15B, a portion surrounded by a dotted line 1510 is one pixel and includes a source signal line 1511, a gate signal line 1512, an electric current supply line 1513, a switching TFT 1514, an TFT 1515 for driving an EL element, a holding capacitance 1516, and an EL element 1517.
Next, the action of the active matrix type spontaneous light emitting device will be described with reference to FIG. 15B. First, when the gate signal line 1512 is selected, a voltage is applied to the gate electrode of the switching TFT 1514 to bring the switching TFT 1514 into conduction and then the signal (voltage) of the source signal line 1511 is accumulated in the holding capacitance 1516. Since the voltage of the holding capacitance 1516 becomes the voltage VGS between the gate and source of the TFT 1515 for driving an EL element, an electric current responsive to the voltage of the holding capacitance 1516 flows through the TFT 1515 for driving an EL element and the EL element 1517. As a result, the EL element 1517 emits light.
The luminance of the EL element 1517, that is, the amount of electric current flowing through the EL element 1517 can be controlled by the VGS of the TFT 1515 for driving an EL element. The VGS is the voltage of the holding capacitance 1516 and the signal (voltage) applied to the source signal line 1511. In other words, by controlling the signal (voltage) applied to the source signal line 1511, the luminance of the EL element is controlled. Finally, the gate signal line 1512 is brought out of a selected state and the gate of the switching TFT 1514 is closed to bring the switching TFT 1514 out of conduction. At that time, the charges accumulated in the holding capacitance 1516 are held. Therefore, the VGS of the TFT 1515 for driving an EL element is held as it is and an electric current corresponding to the VGS continues to flow through the EL element 1517 via the TFT 1515 for driving an EL element.
As to driving the EL element, results of researches are reported in SID99, page 372, “Current Status and Future of Light-Emitting Polymer Display Driven by Poly-Si TFT”; ASIA DISPLAY 98, page 217, “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”; and Euro Display 99 Late News, page 27, “3.8 Green OLED with Low Temperature Poly-Si TFT”.
Next, the mode of the gradation display of the EL element 1517 will be described. An analog gradation mode in which the luminance of the EL element 1517 is controlled by the voltage VGS between the gate and source of the TFT 1515 for driving an EL element, as described above, has a drawback that the luminance of the EL element 1517 is susceptible to variations in current characteristics of the TFT 1515 for driving an EL element. In other words, when the current characteristics of the TFT 1515 for driving an EL element are changed, even if the same gate voltage is applied thereto, the value of the electric current flowing through the TFT 1515 for driving an EL element and the EL element 1517 is changed. As a result, this changes the luminance, that is, the gradation of the EL element 1517.
Hence, in order to reduce variations in characteristics of the TFT 1515 for driving an EL element and to obtain a uniform screen, a mode called a digital gradation mode has been invented. This mode is the one in which the gradation is controlled by two states of the absolute value of voltage |VGS| between the gate and source of the TFT 1515 for driving an EL element: one state in which the voltage |VGS| is smaller than a voltage for starting emitting light (the electric current hardly flows) and another state in which the voltage |VGS| is larger than a luminance saturating voltage (nearly maximum electric current flows). In this case, if the voltage |VGS| is made sufficiently larger than the luminance saturating voltage, even if the current characteristics of the TFT 1515 for driving an EL element are varied, the value of electric current comes near to IMAX. Therefore, this can extremely reduce the effect of the variations in the current characteristics of the TFT 1515 for driving an EL element. Since the gradation is controlled by the two states of an ON state (in which the screen is bright because the maximum electric current flows) and an OFF state (in which the screen is dark because the electric current does not flow), as described above, this mode is called a digital gradation mode.
However, in the case of the digital gradation mode, only two gradations can be displayed in this state. Hence, a plurality of technologies have been proposed in which another mode is combined with the digital gradation mode technology to make a multiple-step gradation.
Among the multiple-step gradation modes is a time-gradation mode. The time-gradation mode is the one in which the gradation is produced by changing time during which an EL element 817 emits light: in other words, one frame period is divided into a plurality sub-frame periods and the number or the length of the sub-frame periods during which the EL element 817 emits light is controlled to display gradations.
See FIG. 9. FIG. 9 simply shows a timing chart of a time-gradation mode. This is an example in which a frame frequency is 60 Hz and in which three-bit gradation is produced by the time-gradation mode.
As shown in FIG. 9A, one frame period is divided into sub-frames periods of the number of bits displaying the gradation. Here, since the number of bits displaying the gradation is three, the one frame period is divided into three sub-frame periods SF1, SF2, and SF3. The one sub frame period is further divided into an address period (Ta#) and sustaining (lighting) period (Ts#). The sustaining period in the SF1 is called Ts1. Similarly, the sustaining periods in the SF2 and SF3 are called Ts2 and Ts3. The address periods Ta1 to Ta3 are equal to each other in the respective sub-frame periods because the address period is a time during which an image signal of one frame is written. Here, the sustaining periods are determined at a ratio of the n-th power of 2, like Ts1:Ts2:Ts3=22:21:20=4:2:1. However, even if the ratio of length of the sustaining period is not a ratio of the n-th power of 2, as described above, the gradation can be expressed.
The gradation is displayed by a method of controlling illuminance by changing the total time in which the EL element emits light in one frame period by controlling the EL element in a state where it emits light or in a state in which it does not emit light in the sustaining (lighting) period from Ts1 to Ts3. In this example, as shown in FIG. 9B, the length of light emitting time can be determined in 8 ways (=23), depending on the combinations of light emitting sustaining (lighting) periods, and hence the 8 levels of gradation from 0 (complete black display) to 7 (complete white display) can be displayed. In the time-gradation mode, the gradation can be displayed in this manner. Needless to say, the gradation can be displayed in the same manner also in an spontaneous light emitting device for a color display.
In the case where the number of levels of gradation needs to be increased, it is recommended that the number of divisions in one frame period be increased. In the case where one frame period is divided into n sub-frame periods, the ratio of the lengths of sustaining (lighting) periods becomes like Ts1: Ts2: Ts3: . . . : Ts(n-1): Tsn=2(n-1): 2(n-2): . . . : 21: 20, and hence the 2n levels of gradation can be displayed. In this connection, as to the order of the sub-frame periods, SF1 to SFn, may appear at random.
Here, problems relating to the spontaneous light emitting device using the spontaneous light emitting element such as an EL element or the like will be described. As described above, while the EL element emits light, the electric current is always supplied to the EL element and hence flows therethrough. Therefore, if the EL element emits light for a long time, the EL element is degraded in its quality, which causes a change in luminance characteristics. In other words, even if an EL element which is degraded and an EL element which is not degraded are supplied with the same voltage from the same power source, they are different form each other in luminance.
Describing a specific example, FIG. 10A is a display screen of a personal digital assistant or the like using a spontaneous light emitting device and displays icons for operation 1001 and the like. Usually, in the use of such a device, the ratio of a still picture display as shown in FIG. 10A is large. At that time, if the icons and the like are displayed in brighter color (gradation) than the background, the EL elements in the pixels in the portion where the icons are displayed emit light for a longer time than the EL elements displaying the background and hence are rapidly degraded.
Assuming that the degradation of the EL elements proceeds under such conditions, display examples of the spontaneous light emitting device after degradation are shown in FIG. 10B, C. First, in the case of a black display shown in FIG. 10B, the spontaneous light emitting element including the EL element displays black in the state where a voltage is not applied to the element and thus does not present a problem of degradation when it displays black. In the case of a white display, even if the EL element which is degraded because it emits light for a long time (in this case, the EL element in the portion where the icons and the like are displayed) is supplied with the same current, it can not produce sufficient luminance but produces variations in luminance, as shown by a reference numeral 1011 in FIG. 10C.
Among methods of eliminating variations in luminance is a method of increasing a voltage applied to the degraded EL element. However, conventionally, an electric current supply line is a single wiring in the spontaneous light emitting device and it is not easy to constitute in a pixel portion a circuit for changing a voltage applied to the EL element in a specific pixel of the pixels arranged in a matrix. Further, because the EL driving TFT has variations, as described above, such a correction method is not desirable.
Further, in the spontaneous light emitting device for a color display, the EL elements for displaying red, green, blue are sometimes different from each other in the degrees of luminance and degradation. Although some methods for correcting the variations in luminance caused by these reasons have been proposed, even the pixels of the same color sometimes produce variations in the degree of degradation and luminance and in this case, the above-mentioned methods can not solve these variations.
As another method for solving the problem is also thought a method of using an EL element having characteristics capable of emitting light for a long time, but the life of the EL element in the current state of art is not sufficient. Therefore, the object of the present invention is to provide a spontaneous light emitting device capable of displaying a normal image having no variations in luminance, even if the elements in the screen are degraded.